Question

I’m working on a python/django app which, among other things, syncs data to a variety of other services, including samba shares, ssh(scp) servers, Google apps, and others. As such, it needs to store the credentials to access these services. Storing them as unencrypted fields would be, I presume, a Bad Idea, as an SQL injection attack could retrieve the credentials. So I would need to encrypt the creds before storage – are there any reliable libraries to achieve this?

Once the creds are encrypted, they would need to be decrypted before being usable. There are two use cases for my app:

• One is interactive – in this case the user would provide the password to unlock the credentials.
• The other is an automated sync – this is started by a cron job or similar. Where would I keep the password in order to minimise risk of exploits here?

Or is there a different approach to this problem I should be taking?

Answer 1

First storing on a server credentials enough to login to a multitude of systems looks like a nightmare. Compromising code on your server will leak them all whatever the encryption.

You should store only the credentials that would be necessary to perform your task (i.e. files sync). For servers you should consider using synchronization server like RSync, for Google the protocols like OAuth etc. This way if your server is compromised this will only leak the data not the access to systems.

Next thing is encrypting these credentials. For cryptography I advise you to use PYCrypto.

For all random numbers you would use in your cryptography generate them by Crypto.Random (or some other strong method) to be sure they are strong enough.

You should not encrypt different credentials with the same key. The method I would recommend is this:

1. Your server should have it’s master secret M (derived from /dev/random). Store it in the file owned by root and readable by root only.
2. When your server starts with root privileges it reads the file into memory and before serving clients drops it’s privileges. That’s normal practice for web servers and other demons.
3. When you are to write a new credential (or update existing one) generate a random block S. Take the first half and calculate hash K=H(S₁,M). That would be your encryption key.
4. Use CBC mode to encrypt your data. Take the initialization vector (IV) from S₂.
5. Store S alongside with encrypted data.

When you need to decrypt just take out S create the K and decrypt with the same IV.

For hash I would advise SHA1, for encryption — AES. Hashes and symmetric cyphers are fast enough so going for larger key sizes wouldn’t hurt.

This scheme is a bit overshot in some places but again this wouldn’t hurt.

But remember again, best way to store credentials is not to store credentials, and when you have to, use the least privileged ones that will allow you to accomplish the task.